Voltage amplitude control



May 7, 1957 N. L. COHEN 2,791,741

VOLTAGE AMPLITUDE CONTROL Filed March 8, 1954 W TEH-E I N V EN TOR.NATHAN/EL L. COHEN f/TTORNEYS United States Patent "ice VOLTAGEAMPLITUDE CONTROL Nathaniel L. Cohen, New Milford, N. J., assignorto-the United States of America as represented by the Secretary of theNavy A plication March 8, 1954,'Sen'al No. 414,913 1 Claim. (Cl. 323*30)This invention relates to a circuit for voltage amplitude control andmore particularly to a circuit for voltage amplitude control orlimiting, relying for its operation on a gated beam tube, adapted toeliminate voltage amplitude variations from a frequency modulationsignal or the like, and in addition adapted to generate rectangularpulses.

it is well known that the advantages of frequency modula-t-ion are bestutilized when the received FM signal waveform is limited to precludeamplitude variations. The gated beam tube has several characteristicswhich makes it admirably suited for eliminating amplitude variationsfroma frequency modulation signal. It is also useful for generatingrectangular pulses. For a discussion of a gated beam tube of aparticulartype known by the commercial marking G l 3N6, reference is made toProeeedin-gs of .the :National Electronics Conference, volume September2 6, 27, 28, 1949, pages 4 08 to 426, and Electronics, volume 2 3,Number 2, February 1950, page 82.

Generally, the gated beam tube includes an electron gun having afocusing means. By designing the lens of the electron gun so that itsuitably shields the cathode from other associated electrodes it ispossible to maintain a constantbeam. current regardless of changes inthe poten'tials of the other electrodes. In the path of the beamcurrent, comprising a stream of electrons issuing from the electron gun,there is disposed an accelerator electrode formed with a suitablerestricted slot or aperture. Beyond the slot in theaccelerator-electrode and in the path of the beam current there isdisposed a signal grid otherwise called the control grid. If thepotential of the signal grid is made suiiiciently negative, no beamcurrent passes through the signal .grid. correspondingly, if thepotential of the signal grid is raised sufficiently positive, a maximumpercentage of the bearn current passes through the signal grid and ontoward the subsequent electrodes. This generally applies to all electrontubes, hereinafter called tubes, having control grids. However, what isremarkable in the case of the gated beam tube is that the necessarypotential swing on the signal grid forproduc- 'ing this changeis'verystnall. A slight negative potential on the signal grid causes thelatter to act as a perfect barrier to the beam current; a slightpositive potential on the signal grid causes the latter to beeffectively transparent to the beam current and the aforementionedmaximum percentage of the beam current passes through the signal grid ontoward the subsequent electrodes in the path of the beam current. It isthis sharp transitional change, namely, that the beam current passingthrough the signal grid varies between zero and a maximum under thecontrol of a small change in potential of the signal grid, which is oneof the important characteristics relied upon for the proper operation ofthe circuit of this invention.

Due to the fact that the electrons comprising the beam current in thegated beam tube do not return along the same path that they follow inmoving from the cathode 2,791,741 Patented May 7, 1957 to the signalgrid substantially all the beam current that is turned back by thesignal grid when its potential swings slightly negative is captured bythe accelerator electrode. Therefore the current in the circuit of theaccelerator electrode also varies sharply between minimum and maximum.

The gated beam tube is further provided with a second grid, generallyknown as the quadrature grid, followed by an anode (or plate) both ofwhich are in the path of the beam current issuing from the electron gun.

The quadrature grid affects the tube operation in a manner similar tothe signal grid. Within narrow limits, the amount of anode current iscontrolled by the quadraturegn'd. However, if the potential of thequadrature grid is made .sufiiciently positive, it no longer exercisesany control over the anode current since the current can never riseabove a certain amount which is a fixed percentage of the beam current;the amount of beam current issuing from the electron gun is fixed.

Both the signal grid and the quadrature grid are adapted to cut off theanode current.

(The characteristics of the gated beam tube utilized are (l) theconstant beam current issuing from the cathode, (2) thevsteep rise ofplate current in the signal grid-plate transfer characteristic from zeroto a maximum due to a very slight rise in signal grid potential (3) thesteep rise of plate current in the quadrature grid-plate transfercharacteristic due to a slight rise in quadrature grid voltage.

The section of the gated beam tube comprising the cathode, signal grid,and accelerator acts in a manner similar to that of a dynatron. Thesection including the quadrature grid and anode acts like a tiiode.

The circuit of this invention adapts the gated beam tube for veryeffective use as a limiter. With no bias on the signal grid and with apositive bias on the quadrature grid, a signal grid potential swing inthe general vicinity of one volt causes a considerable variation inaccelerator current which is accompanied by a reverse considerablevariation in accelerator voltage. The variation in accele'r'ator voltageis coupled through a condenser to the quadrature grid to cause the anodecurrent to be either reduced to zero or raised to a maximum. Theresulting voltage swing at the anode is coupled to another stage througha coupling condenser.

An object of this invention is to provide a voltage amplitude control.

Another object is to provide a voltage amplitude control adapted tofunction as a limiter.

Another object is to provide a voltage amplitude control adapted tofunction as a limiter for generating rectangular pulses.

A further object of this invention is to provide a volt age amplitudecontrol for use in eliminating amplitude variations from frequencymodulation signals or the like.

A further object is to adapt the characteristics of a 6BN6 gated beamtube to voltage amplitude control.

Other objects and many of the attendant advantages of thisinvention'will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein:

Fig. 1 is a graph showing a signal grid-plate transfer characteristic ofa 6BN6 gated beam tube, for three levels of quadrature grid potential,

Fig. 2 is a graph showing a signal grid-accelerator transfercharacteristic of 6BN6 gated beam tube for one particular potential onthe quadrature grid, and

Fig. 3 is a schematic diagram of a preferred circuit according to thisinvention.

There is shown in the schematic diagram a gated beam tube 12. Includedwithin the envelope of the gated beam tube 12 is a cathode 14. Connectedto the cathode and at the same potential as the cathode are a focuselectrode 15. The tube 12 further includes a signal grid 16, a lens 17surrounding the signal grid, an accelerator electrode 18 having ascreened opening, a quadrature electrode 22, an anode 24, and a shield25. There is no bias provided between the cathode 14 and the signal gridi6. The input signal voltage is applied to the circuit at the terminals26 and 28. A potential is provided between the cathode 14 and theaccelerator electrode 18 by means of the battery 32. In the circuit withthe battery 32 between the accelerator electrode 13 and the cathode 14is a resistor 34. A voltage dividing means such as a potentiometer'36having a tap 37 is connected in shunt across the battery 32 to providefor a selectively variable bias on the quadrature grid 22. In seriescircuit between the quadrature grid 22 and the tap 37 of potentiometer36 is a resistor 38. A coupliug condenser 42 is directly connected tothe accelerator electrode 1% and the quadrature grid 22 at the ends ofthe resistors 34 and 38, respectively. Resistor 38 and condenser 42provide a difierentiator circuit for the quadrature grid 22. 'One end ofload resistor 44 is connected to the anode (or plate) 24 and the otherend 'of the load resistor 44 is connected to the positive terminal ofthe plate supply 46. The other terminal of the plate supply 46 isconnected to ground. A coupling condenser 48 is provided for couplingthe output of the circuit to another stage. In operation, assuming anegative goingportion of an FM signal is being applied to the signalgrid, as the grid 16 goes negative the current in the acceleratorincreases rapidly causing a rapid drop in accelerator voltage. Themagnitude of the drop in accelerator voltage is a function of theaccelerator current and the size of resistor 34. V The drop isconsiderably greater than that on the signal grid. The drop isinstantaneously coupled to the quadrature grid 22 through the resistor38-condenser 42 difierentiator. The magnitude of the drop is snfiicientto cut oi? the plate current causing the potential at the plate to riseto that of battery 46. Assuming that a positive going portion of an FMsignal is applied to the signal grid 16, the immediate result is thatthe current through the accelerator 18 and the resistor 34 decreasesresulting 'in an increase in voltage at the accelerator 18. This changein voltage on the accelerator 18 is communicated to the quadrature grid22 through the resistor 33-condenser 42 coupling causing the platecurrent to rise to a maximum and the potential to -drop accordingly.

Change in potential at the quadrature grid 22 acts to reinforce theeffect of the signal grid voltage on the plate current. Therefore,though theamplitude of the original input signal at 26-23 may vary moreor less, the output amplitude isfixed. The transition from zero tomaximum plate current takes place over a very limited range of signalgrid voltage swing whereby the output plate voltage remains constant forall values of grid voltage above one limit and is zero for all values ofgrid voltage below the other limit. The eifect of the connection fromaccelerator to quadrature grid is to increase the efiectiveness of thesignal grid voltage. Essentially it may be considered a form of positivefeedback. Because of the inherent shieldmg of the quadrature grid on theanode, it is possible to obtain apparent positive feedback efifects fargreater than could normally be obtained in a conventional amplifierwithout reaching instability. In addition, because of the independenceof the plate circuit from the feedback circuit, there is little or noeffect on the output impedance of the device. Hence the frequencyresponse is not noticeably affected. Through proper design of theelements of this invention, it is possible to convert an input signalinto a series of rectangular pulses corresponding to the former inrepetition rate.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

I claim:

A frequency modulation limiter circuit comprising a gated beam tube,said gated beam tube including a cathode, a signal grid, an acceleratorelectrode, a quadrature grid, and a plate; a coupling condenserconnected between the quadrature grid and the accelerator electrodewhereby variations of the potential of the accelerator electrode areimmediately transferred to the quadrature gr a first direct currentpower supply having a positive sue a negative terminal, the negativeterminal of said first battery connected to said cathode; a firstresistor, one end of said first resistor connected to the positiveterminal of said battery and the other end of said resistor connected tothe accelerator electrode; a voltagedivider connected in shunt acrosssaid first direct current power supply and having an adjustable tap; asecond resistor, one end of said second resistor connected to thequadrature grid and the other end of said second resistor connected tothe tap of said voltage divider; a second direct current power supplyhaving a positive terminal and a negative terminal, the negativeterminal of said second. supply connected to said cathode; a thirdresistor constituting a plate load resistor, one end of said thirdresistor connected to said plate and the other end of said thirdresistor connected to the positive terminal of said second batterywhereby when a signal whose voltage range includes the eifective signalgrid voltage range is applied to the signal grid, current in theaccelerator circuit changes rapidly causing an opposite change in theaccelerator voltage, the latter being coupled by the RC networkconsisting of said condenser and said second resistor to the quadrauturegrid, the action of the quadrature grid being such as to reinforce theeffectrof the signal grid voltage on the plate current to cause theoutput plate voltage to be constant at one limit for all values ofsignal grid voltage above a certain value and to be constant at anotherlimit for all values of signal grid voltage below a certain value.

References Cited in the file of this patent UNITED STATES PATENTS2,404,919

OTHER REFERENCES PublicationAdler: A Gated Beam Tube; Electronics, vol.23, No. 2, February 1950, pp. 82435.

